1. Field of the Invention
The present invention relates generally to audio/video systems and methods and, more particularly, to different ways to control audio rate adjustment that prevents underflow or overflow.
2. Introduction
In an MPEG audio/video system, the MPEG transport stream is transmitted from the head end (via either cable or satellite). The decoder in the receiver derives its timing (time base) from the MPEG transport stream program clock reference (PCR) and uses it as its display timing. This ensures that the display timing is locked to the incoming MPEG transport stream, thereby providing a stable system with no audio/video data underflow or overflow. Details of the MPEG system are provided in ISO/IEC 13818-1, which is incorporated herein by reference in its entirety.
One example of an MPEG audio/video decoder system is shown in FIG. 1. As illustrated, transport processor 110 uses the PCR from the MPEG transport stream to generate the time base. As a result, this time base is locked to the input MPEG transport stream. The generated time base is used to generate the audio sample rate and the video display rate via audio numerical controlled oscillator (NCO) 122 and video NCO 132, respectively. The audio sample rate clock generated by audio NCO 122 is used to drive audio display 126, which displays the decoded audio PCM data generated by audio decoder 124. Similarly, the video sample rate clock generated by video NCO 132 is used to drive video display 136, which displays the decoded video data generated by video decoder 134. As both audio display 126 and video display 136 are driven by a sample rate clock derived from the input MPEG transport stream, the system is stable and there is no audio or video data underflow or overflow.
In MPEG dual real-time audio/video systems, there is a desire to display audio in a time base that is different from its source. For example, in a TV that has picture-in-picture (PIP) capability, the user might choose to switch the audio from the main audio to the PIP audio or vice versa without switching the corresponding video. When the user chooses main audio along with the main video, the display time base matches the audio source time base and the audio system is stable with no overflow or underflow of audio data. This is the scenario illustrated in FIG. 1. If the user chooses to listen to PIP audio along with the main video, however, the PIP audio will be displayed at the main time base, which can be different from the PIP time base by a few hundred parts per million (PPM). An audio PPM rate adjustment would then be needed for this usage mode to prevent audio data from entering into an underflow or overflow condition. What is needed therefore is a system and method that can control audio rate adjustment between source and display time bases.